The invention is directed to spray apparatus of the type which is generally described as electrostatic spray guns or as electrostatic spray coating guns. The electrostatic spray apparatus may be designed either for spraying on an article to be coated coating material in the form of a powder or coating material in the form of a liquid. The spray apparatus may be designed as hand held spray gun which includes a handle, or it may be designed as an automatic spray gun adapted to be supported by, for example, a stationary support, a reciprocator or an industrial robot. All of these options are included in the present invention.
An electrostatic spray gun according to the prior art is shown, for example, in U.S. Pat. No. 4,196,465. The spray gun serves to spray coating powder on an article. The spray gun has a barrel to which a handle can be attached or which may be mounted on a known carrier machine. A powder duct extends through the spray gun barrel from a rear end of the barrel to a spray nozzle on a front end of the barrel. The spray nozzle and the spray gun barrel are stationary relative to each other. The coating power is pneumatically fed by a compressed air flow and is atomized at the spray nozzle to form a powder cloud which flows onto the article to be coated. The spraying or atomization of the powder takes place at the spray nozzle through a nozzle effect and/or a diffuser effect.
One or more high voltage electrodes are disposed near the nozzle in the powder flow path. The electrodes are electrically connected to the high voltage output of a voltage generator. A high voltage applied to the electrodes may be in the range of between 1 KV and 170 KV for imparting an electrostatic charge to the sprayed powder relative to the article being coated. The voltage generator may be accommodated in the spray gun barrel. The voltage generator includes, for example, a converter or oscillator for converting a low voltage DC to AC, a transformer and a cascade circuit which are connected together. A low voltage input to the generator is adapted for connection to an external low voltage power source by means of a electric wire or cable. The low voltage may be DC or may be rectified from a low level AC voltage to form a DC low voltage. The oscillator converts this to an AC voltage which is stepped up to an intermediate voltage AC signal by the transformer. The cascade circuit then multiplies the intermediate level AC voltage to generate the desired high voltage. In accordance with a modified embodiment, the generator can be arranged outside of the spray gun and connected to the high voltage electrodes in the spray gun by way of a high voltage cable.
European patent application EP-A 0,513,626 teaches an electrostatic spray gun for spraying liquid coating material on an article in which the spraying or atomization of the coating liquid at the nozzle is aided by additional atomization air. If the coating liquid is fed to the nozzle at a relatively high pressure, the liquid can be atomized at the spray nozzle in the same manner as coating powder. The additional atomizing air is used to allow a reduction in the feed pressure of the coating liquid required for atomization. The atomizing air may be high pressure air with a low flow volume, or it may be low pressure air with a high flow volume, or it may be within an in between range. A high volume low pressure air flow in a spray gun is referred to as HVLP. To form a fan type spray stream of coating liquid, it is known to compress the atomized coating liquid envelope into a flatted fan shaped envelope by means of shaping air directed at opposite sides of the atomized liquid envelope. Coating liquid spray guns of this type may be provided with one or more high voltage electrodes to electrostatically charge the coating material relative to the article being coated. This results in a better coating quality and in a better coating efficiency since the electrostatic charge draws the coating particles towards the article being coated.
The art also teaches that for both coating powder and coating liquid, a fan shaped jet or envelope can be imparted to the sprayed material through the use of a nozzle with a discharge orifice having a slotted cross sectional shape.
The prior art such as U.S. Pat. No. 5,353,995 and European patent application EP-A 0,410,717 also teaches that the stationary spray nozzle as described above may be replaced with a rotating atomizer bell both in powder spray coating apparatus and in liquid spray coating apparatus. It has been demonstrated that a major advantage from using a rotary atomizer bell is that a more even coating thickness can be produced on flat surfaces. The coating thickness is more even due to the shape of the powder cloud emitted from the atomizer. The rotation of the atomizer bell generates a powder cloud having a relatively large diameter, for example, of 0.5 m, with homogeneous powder particle distribution.
The cloud of atomized powder has little forward velocity and is drawn to the article being coated through electrostatic attraction. The kinetic energy of the powder particles in the powder duct of the spray apparatus is distributed across the very much larger cross sectional area of the larger powder cloud. As a result, the powder particles receive less forward moving movement as they are atomized. This allows the particles to better follow the electric field lines extending from the high voltage electrode to the grounded electric conductor nearest the electrode, which is normally the closest points on the article being coated. As a consequence, the powder fails to penetrate and uniformly cover surfaces in cavities on the article being coated.
It is generally known that nozzles produce a forwardly directed fan jet have different coating characteristics from rotary atomizers. The film thickness distribution on the article being coated is frequently mediocre, whereas the penetration of the coating particles into cavities of the article being coated is very good. It would be desirable to produce a spray gun which creates the coating thickness uniformity of a rotary atomizer bell and which produces a spray which will penetrate cavities on the article being coated as with a stationary atomizer nozzle.